R help - Apr 2013 - Left join in R

I have never used the data.table package.  I am trying to do the following
SQL left join  in R

create table all as select a.*
                            from dates b left outerjoin activitycount a on
                            a.tdate=b.tdate 
                         and a.activity=b.activity

I found the following example but cannot figure out  the syntax:

le <- Y[X]
mallx <- merge(X,Y, all.x = T)
# the column order is different so change to be the same as `merge`
setcolorder(le, names(mallx))
identical(le,mallx)
# [1] TRUE

Can anyone help?  Thanks.
                      



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